Line a rimmed baking sheet with parchment paper or tin foil and preheat the oven to 350℉ and set a rack to the center of the oven.
Check the salmon for any bones and cut it into two equal pieces. Brush both sides with a little oil, and season lightly with salt and pepper.
Trim the woody stalks from the bottom of the asparagus, toss with the remaining oil, and season with salt and pepper.
Place the salmon skin side up near the edges of the baking sheet and spread the asparagus out in the center. Place it in the preheated oven and cook for 15 minutes.
While the salmon is baking, melt the butter and mix in the mustard, lemon juice, and dill. Taste the sauce and add salt and pepper if desired.
After fifteen minutes, check the salmon. It should be firm to the touch but starting to flake under pressure. The interior should look pink but firm. If it isn’t ready, return it to the oven for another five minutes or until done.
Transfer one piece of salmon and half the asparagus to a plate and serve with the butter sauce.
Cool the remaining piece of salmon and asparagus and store in airtight containers. The extra asparagus can be used in salads and soups throughout the week.
Notes
*You can substitute broccolini or green beans for the asparagus if desired.